When it is desired to coat a leather, a woven or non-woven fabric, or any other flexible material, with a film of polyurethane-polyurea, it is difficult to obtain a product which has both satisfactory resistance to repeated flexing and at the same time satisfactory resistance to abrasion, because these two important characteristics are generally antagonistic.
Another property which often also plays a very important role in the manufacture of polyurethane-polyurea flexible assemblies is scratch resistance. This is particularly the case in the shoe industry. However, the same factors which increase flex resistance are often found to diminish the scratch resistance.
It is known that polyurethane-polyurea elastomers generally possess a high resistance to abrasion when the ratio of urea to urethane is high. The same is true for resistance to scratching. These generalizations are only true, however, when the average molecule weight of the polyol of mixture of polyols is about equal. Unfortunately, the more the urea/urethane ratio is increased (still at constant level of mean molecular weight of polyol or polyols) the more the rigidity of the elastomer is increased and therefore the more the flex-resistance diminishes.
Another property which is very difficult to evaluate, but which is of extreme importance for all coatings on flexible substrates is the "feel" or "hand" which likewise depends to a high degree on the urea/urethane ratio. The choice of compositions has consequently always been a matter of compromise.
It is also well known that polyester groupings (with the exception of those derived from E-caprolactone) are deleterious to resistance to aging in humid atmospheres.
The use of a prepolymer having simultaneously polyether groupings and polyester groupings achieves results superior to those obtained with polyesters alone, but the resistance to aging in humid atmospheres is always superior when polyol polyethers are used alone.